ETA Event Tree Analysis is a part of maintenance

EVENT TREE ANALYSIS
Event tree analysis evaluates potential accident
outcomes that might result following an equipment
failure or process upset known as an initiating event.
It is a “forward-thinking” process, i.e. the analyst
begins with an initiating event and develops the
following sequences of events that describes
potential accidents, accounting for both the successes
and failures of the safety functions as the accident
progresses.

Guidelines
1. Identify an initiating event of interest.
2. Identify the safety functions designed to
deal with the initiating event.
3. Construct the event tree.
4. Describe the resulting accident event
sequences.

Step 1 Identify the initiating event
• system or equipment failure
• human error
• process upset
[Example]
“Loss of Cooling Water”
to an Oxidation Reactor

Step 2 Identify the Safety Functions
Designed to Deal with the Initiating
Event
• Safety system that automatically respond to
the initiating event.
• Alarms that alert the operator when the initi
ating event occurs and operator actions desi
gned to be performed in response to alarms
or required by procedures.
• Barriers or Containment methods that are in
tended to limit the effects of the initiating e
vent.

Example
• Oxidation reactor high temp. Alarm alerts
operator at temp T1.
• Operator reestablish cooling water flow to the
oxidation reactor.
• Automatic shutdown system stops reaction at
temp. T2. T2 > T1
These safety functions are listed in the order in
which they are intended to occur.

Step 3: Construct the Event Tree
a. Enter the initiating event and safety functions.
SAFETY
FUNCTION
Oxidation reactor
high temperature
alarm alerts operator
at temperature T1
Operator
reestablishes
cooling water flow
to oxidation reactor
Automatic
shutdown system
stops reaction at
temperature T2
INITIATING EVENT:
Loss of cooling water
FIRST STEP IN CONSTRUCTING EVENT TREE

b. Evaluate the safety functions.
SAFETY
FUNCTION
Oxidation reactor
high temperature
at temperature T1
Operator
reestablishes
cooling water flow
Automatic
shutdown system
stops reaction at
temperature T2
INITIATING EVENT:
REPRESENTATION OF THE FIRST SAFETY FUNCTION
Success
Failure

b) Evaluate the safety functions.
SAFETY
FUNCTION
Oxidation reactor
high temperature
at temperature T1
Operator
reestablishes
cooling water flow
Automatic
shutdown system
stops reaction at
temperature T2
INITIATING EVENT:
REPRESENTATION OF THE SECOND SAFETY FUNCTION
Success
Failure
If the safety function does not affect the course of the
accident, the accident path proceeds with no branch pt to
the next safety function.

Step 3: b. Evaluate safety functions.
SAFETY
FUNCTION
Oxidation reactor
high temperature
at temperature T1
Operator
reestablishes
cooling water flow
Automatic
shutdown system
stops reaction at
temperature T2
INITIATING EVENT:
COMPLETED EVENT TREE
Success
Failure
Completed !

Step 4: Describe the Accident Sequence
SAFETY
FUNCTION
Oxidation reactor
high temperature
at temperature T1
Operator
reestablishes
cooling water flow
Automatic
shutdown system
stops reaction at
temperature T2
INITIATING EVENT:
ACCIDENT SEQUENCES
Success
Failure
Safe condition,
return to normal
operation
Safe condition,
process shutdown
Unsafe condition,
runaway reaction,
operator aware of
problem
Unstable condition,
process shutdown
Unsafe condition,
runaway reaction,
operator unaware
of problem
B
A
C D
A
AC
ACD
AB
ABD

Reactor
TIA
TIC
Alarm
at
T > TA
Figure 11-8 Reactor with high
temperature alarm and temperature
controller.
Cooling Coils
Thermocouple
High Temperature Alarm
Temperature
Controller
Reactor Feed
Cooling Water Out
Cooling
Water In

Safety Function:
Identifier: B C D E
Failures/Demand: 0.01 0.25 0.25 0.1
High Temp
Alarm Alerts
Operator
Operator
Notices
High Temp
Operator
Re-starts
Cooling
Operator
Shuts Down
Reactor Result
Shutdown = 0.2227 + 0.001688 + 0.005625 = 0.2250 occurrences/yr.
Runaway = 0.02475 + 0.0001875 + 0.0000625 = 0.02500 occurrences/yr.
Figure 11-9 Event tree for a loss of coolant accident for the reactor of Figure 11-8.
Initiating Event:
Loss of Cooling
1 Occurrence/yr.
A
1
A
0.7425
AD
0.2227
ADE
0.02475
AB
0.005625
ABD
0.001688
ABDE
0.0001875
ABC
0.001875
ABCD
0.0005625
ABCDE
0.0000625
0.99
0.01
0.2475
0.001875
0.000625
0.0075
0.0025
Continue Operation
Shut Down
Runaway
Continue Operation
Shut Down
Runaway
Continue Operation
Shut Down
Runaway

Safety Function
0.01 Failures/Demand
Initiating
Event
0.5 Occurrences/yr.
Success of Safety Function
(1-0.01)*0.5 = 0.495 Occurrence/yr.
Failure of Safety Function
0.01*0.5 = 0.005 Occurrence/yr.
Figure 11-10 The computational sequence across a safety function in an
event tree.

Safety Function:
Identifier: B C D E F
Failures/Demand: 0.01 0.25 0.25 0.01 0.1
High Temp
Alarm Alerts
Operator
Operator
Notices
High Temp
Operator
Re-starts
Cooling
Operator
Shuts Down
Result
Shutdown = 0.2450 + 0.001856 + 0.00001688 + 0.0006187 = 0.2475 occurrences/yr.
Runaway = 0.0002475 + 0.000001875 + 0.000000625 = 0.0002500 occurrences/yr.
Figure 11-11 Event tree for the reactor of Figure 11-8. This includes a high temperature shutdown system.
Initiating Event:
Loss of Cooling
1 Occurrence/yr.
A
1
A
0.7425
0.99
0.01
0.2475
0.001875
0.000625
0.00750
0.0025
Continue Operation
Shut Down
Shut Down
Runaway
Operator
Shuts Down
Reactor
AD
0.2450
ADE
0.002228
ADEF
0.0002475
AB
0.005625
ABD
0.001856
ABDE
0.00001688
ABDEF
0.000001875
ABC
0.001875
ABCD
0.0006187
ABCDE
0.00000563
ABCDEF
0.000000625
0.002475
0.00001875
0.00000675
Continue Operation
Shut Down
Shut Down
Runaway
Continue Operation
Shut Down
Shut Down
Runaway

ETA Event Tree Analysis is a part of maintenance

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